The goal of the proposed study is to understand how thyroid hormone and glucocorticoids act to increase mitochondrial RNA levels. Preliminary experiments show that in HTC cells, a rate liver carcinoma cell line, administration of these hormones increases the levels of several mitochondrial RNA transcripts. First, the effects of these hormones on mitochondrial RNA levels will be better characterized; next, the locus at which these hormones act to increase mitochondrial RNA levels will be determined; and finally, the signal mediating the increase in mitochondrial RNA levels will be identified. The levels of different mitochondrial RNAs from control and treated HTC cells will be compared using Northern transfer or solution hybridization techniques; specific radiolabeled cDNAs or CRNAs will be used as probes. Mitochondrial DNA levels will be measured to control for hormonal effects on mitochondrial DNA replication. Thyroid hormone and glucocorticoid hormone dose response relationships, time course of response and order of analog potency will be tested to better characterized the mitochondrial response. The ability of these hormone treatments to increase mitochondrial RNA levels in primary cultures of rat hepatocytes and in intact rats will be compared with the effects of HTC cells. Next, the site at which the hormones act to increase mitochondrial RNA levels will be determined. The dependence of the response on ongoing nuclear and mitochondrial transcription and translation will be tested. Hormonal effects on mitochondrial transcription rate and RNA half-life will be measured to determine whether increased synthesis or decreased degradation is the mechanism for increasing mitochondrial RNA levels. Finally, an in vitro assay in which the hormonally mediated increase in RNa levels can be reconstituted will be developed to identify the signal mediating the increase in mitochondrial RNA levels. Isolated mitochondria and cytosolic fractions from treated and untreated HTC cells will be used in the isolation and identification of the components that are responsible for mediating the increased mitochondrial RNA levels.